Anatomical Note.—The five lumbar, the five sacral and the four coccygeal vertebrÆ constitute the lumbar and sacro-coccygeal section of the vertebral column.

The sacrum, formed by the union of the five sacral vertebrÆ, appears in the adult in the form of a bone that narrows rapidly from above downward in a general curve whose convex side is turned inward. The coccyx has the importance of being a real and actual caudal appendage, reduced in man to its simplest anatomical expression. On each side of the sacrum the two ossa innominata or hip-bones are attached, constituting a sort of massive girdle (cintura pelvica), serving as point of attachment for the lower limbs, while at the same time it sustains the entire weight of the body and the abdominal viscera. These two bones are made up of three separate parts: an upper part, very broad and rather thin (the ilium, which constitutes the flank or hip), one in front (the os pubis), and a third behind, quite massive, and shaped like the letter V (the ischium). The two ossa innominata and the os sacrum form the pelvis or pelvic basin, a broad cavity with bony walls that are by no means complete, within which are a portion of the digestive organs and a considerable part of the organs belonging to the genito-urinary system. The pelvis supports the vertebral column and is in turn supported by the lower limbs, in quite marvellous equilibrium.

The maximum sexual differences of the skeleton are in relation to the pelvis; in woman the iliac bones form a far ampler basin; in man, the pelvis is higher and more confined and formed of more solid bones; but it is not broader. But where the difference is most apparent is in the pelvic aperture (see Fig. 127) which divides the pelvis into two parts, the upper or great pelvis and the lower or small pelvis. This aperture has distinguishing marks that differ widely between the sexes; in woman it is rounder, in man it is more elongated from front to back and is narrowed toward the pubis. One of the most important points of measurement in anthropology and in obstetrics is the extreme anterior apex of the superior border of the ilium or crista iliaca antero-superior. The woman in whom this dimension (the bis-iliac) is less than 250 millimetres cannot give birth naturally; similarly the woman who has a prominent os pubis (due to rachitis) will owe the attainment of maternity to the intervention of surgery, and perhaps even of the CÆsarean operation.

There are also many ethnical differences in the pelvis: brachycephalics (the mongolian race) have a broader and shallower pelvis than the dolichocephalics, who, on the contrary, have a deeper and narrower pelvis (the negroes). The same thing is met with, notwithstanding its intermixture, in our own race: blond, brachycephalic women have a wider pelvis than brunette, dolichocephalic women.

Accordingly, cranium, thorax and pelvis correspond in one and the same ethnic type.

The abdomen extends from the arch of the diaphragm to the lower extremity of the pelvis. It contains all the viscera of alimentation: the digestive system together with the glands belonging to it; the liver and pancreas, besides the renal system and, in women, the organs of generation (uterus and ovaries). The diaphragmatic arch, having its convex side uppermost, enters the thoracic frame as far as the first dorsal vertebra. The intestinal mass is more noticeable and prominent in persons having a narrow pelvis; in children, for example, the abdomen is very prominent.

Growth of the Pelvis.—In the skeleton of the new-born child the pelvis differs from that of the adult in two particulars: height and direction. The pelvis is low in the new-born child and higher in the adult. The central axis is more oblique from front to back (in the higher mammals the axis of the pelvis is almost central); in the adult, on the contrary, this axis tends to straighten up, to the point of becoming nearly vertical, in relation, that is, to the erect position of man. Hence in the course of growth the pelvis not only becomes proportionally higher, but it undergoes a rotary movement around the cotyloid axis; this movement has the effect of elevating the pubis and bringing the ischium forward.

The vertebral column rests upon the sacrum, which is the retro-cotyloid portion of the pelvis, and its pressure tends mechanically to straighten the pelvis (see diagram, Fig. 128). This process of straightening has certain limits, and is dependent upon the form of curvature of the vertebral column; if this is exaggerated, as in lordosis, the weight is thrown further forward, almost over the cotyles; consequently, the elevation of the pelvis is not properly accomplished (low pelvis found in lordotics). If, on the contrary, the lumbar curvature is wanting or reversed (kyphosis), the pressure of the column is thrown backward and the straightening up of the pelvis is exaggerated (high pelvis found in kyphotics). Independently of pathological deformities, there are various forms of lumbar curvature in the vertebral column that are normal oscillations, or oscillations acquired through adaptation.

An exaggerated lumbar curvature or saddle-back is found in children accustomed to carry heavy loads upon their shoulders; a diminished curvature is found in children constrained to remain in a sitting posture for many hours a day. The sitting posture tends to cancel the lumbar inward curve; consequently, while children are in school they are promoting the elevation of their pelvis.

The elevation of the pelvis proceeds rapidly at the fifteenth year, during puberty, when the muscular masses become more solid.

A woman is not fitted for motherhood, even if physically developed, so long as her pelvis has not rotated normally. But if the rotation is exaggerated (due to prolonged sitting posture during years of growth), this is very unfavourable to normal childbirth. In rickets, associated with kyphosis, there is a form of exaggerated rotated pelvis (pubis high). The laborious "modern" childbirth, and the dangerous childbirth in the case of women who have devoted much time to study, must be considered in connection with these artificial anomalies. Free movement and gymnastics have for this reason, in the case of women, an importance that extends from the individual to the species.